PROJECT SUMMARY: Vertebrate photoreceptors are an elegant example of form being finely tuned to support function. These primary sensory neurons are linearly organized into a series of morphologically and functionally distinct compartments. All of the compartments contribute in different ways to the maintenance and signaling activity of this neuron. For instance, photons are captured in the outer segment, the synapse communicates that event to downstream neurons, and the inner segment, typically thought of as the housekeeping compartment, houses the ion channels, pumps, and transporters needed to set and maintain the circulating current that is ultimately used to communicate the presence or absence of light. The polarized trafficking of select ion channels to the different photoreceptor compartments is well recognized as essential for the health and function of this cell. Yet, the mechanisms controlling the subcellular trafficking and localization of ion channels in this cell or for that matter, most others, is poorly understood at best. The overarching goal of this project is to identify the mechanisms that control polarized protein trafficking in photoreceptors. In this proposal we are building on the knowledge we gained in our earlier studies of HCN1, a hyperpolarization activated channel that filters light responses and is essential for vision in bright light. Aim 1 probes how the assembly status and permissiveness of HCN1 to leave the ER is coordinated. This aim also tests if the mechanisms controlling HCN1 processing are used to regulate Kv2.1/Kv8.2, a related ion channel also found within the inner segment that when absent results in aberrant signaling and cone dystrophy. Aim 2 probes the function and mechanism of a second trafficking signal within HCN1 that we propose is required, at least in part, to recruit the protein coat needed for the generation of HCN1-bearing transport vesicles. A suite of genetic, biochemical, imaging, and physiological tools are used. Altogether, this work will reveal fundamental mechanisms used in the polarized trafficking of photoreceptor proteins, shed light on the contributions made by the early secretory pathway to the regulation of ion channels, and is anticipated to transform current views of the fundamental organization of photoreceptors in health and disease.